I strongly disagree. A gun doesn't make him more serious or darker. I've read a bit of the Batman comics and there's been plenty of great stuff when he didn't use a gun. Plus many superheroes rarely use guns, because they have powers. Batman's power is being smart and using plenty of gadgets (that are NOT guns), along with his fighting style. Hopefully no other Batman movies feature him using a gun, it looks silly and out of character. Just because he had it a long time ago in the comics doesn't justify it much.
Ben Affleck will never be an action star in my view. He's better off doing Kevin Smith movies and/or comedies. Plenty of better names they could've gotten for the role.